Rapid urbanization and expanding city limits lead to the continued emergence of “megacities,” major urban agglomerations that become largely autonomous hubs for customers, talent, investment, wealth creation and growth. Despite their importance, however, there are still significant limits to their ability to address universal problems such as climate change or to pursue other national or global goals. But even this may change: as mayors around the world tackle difficult societal problems, what succeeds in one megacity can quickly become a global standard. Given their scale, this kind of domino effect can affect massive numbers of people.